CVE-2024-35981 Information

May 21, 2024 cve

Description

In the Linux kernel the following vulnerability has been resolved:

virtio_net: Do not send RSS key if it is not supported

There is a bug when setting the RSS options in virtio_net that can break the whole machine getting the kernel into an infinite loop.

Running the following command in any QEMU virtual machine with virtionet will reproduce this problem:

 ethtool -X eth0  hfunc toeplitz

This is how the problem happens:

ethtool_set_rxfh() calls virtnet_set_rxfh()
virtnet_set_rxfh() calls virtnet_commit_rss_command()
virtnet_commit_rss_command() populates 4 entries for the rss scatter-gather
Since the command above does not have a key then the last scatter-gatter entry will be zeroed since rss_key_size == 0. sg_buf_size = vi->rss_key_size;
This buffer is passed to qemu but qemu is not happy with a buffer with zero length and do the following in virtqueue_map_desc() (QEMU function):

if (!sz) virtio_error(vdev irtio: zero sized buffers are not allowed);

virtio_error() (also QEMU function) set the device as broken

vdev->broken = true;
Qemu bails out and do not repond this crazy kernel.
The kernel is waiting for the response to come back (function virtnet_send_command())
The kernel is waiting doing the following :

while (!virtqueue_get_buf(vi->cvq &tmp) && !virtqueue_is_broken(vi->cvq)) cpu_relax();
None of the following functions above is true thus the kernel loops here forever. Keeping in mind that virtqueue_is_broken() does not look at the qemu vdev->broken so it never realizes that the vitio is broken at QEMU side.

Fix it by not sending RSS commands if the feature is not available in the device.

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